SPAWNING AND SETTING OF OYSTERS 
65 
Table 1. — Sex of oysters in Galveston Bay 
Date 
Place 
Males 
Females 
Doubtful' 
1929 
Apr. 15 2 
Bayou, main reef 
11 
0 
Apr. 16 2 
do 
3 
21 
0 
Apr. 17 2 
2 
18 
0 
Apr. 29 
do 
6 
12 
2 
Bayou, middle ground 
7 
9 
0 
]Vla.y 7 
Bayou, main reef 
2 
12 
6 
y 
Bayou, middle ground 
8 
May 9 
Bayou, main reef (top) 
7 
11 
0 
Bayou, main reef (south side) 
5 
0 
Bayou, main reef (north side) 
2 
6 
0 
Bayou, middle ground 
2 
12 
0 
June 12 
do 
5 
12 
5 
June 13 
Deer Island reef 
2 
10 
June 14 * 
Bayou, main reef 
5 
10 
3 
July 11 
Deer Island reef 
2 
7 
3 
July 12 
Bayou, main reef ... 
3 
14 
0 
Total 
51 
178 
70.63 
23 
Percentages 
20.24 
9. 13 
' Specimens which were either spawned out or immature, the sex of which was not readily determined. 
2 Counts made by Dr. Paul S. GaltsofE. 
WATER TEMPERATURE IN RELATION TO SPAWNING 
The daily temperature of the water in Offatts Bayou is given in Figure 9 as the 
average of readings from the thermograph chart at each of the 24 hours daily. The 
bulb of the thermograph was under about 2% feet of water at mean low tide. 
At the end of February and the beginning of March (fig. 9) the temperature was 
between 14° and 15° C., from which it rose, by the middle of March, to 18° to 19° C. 
The major spring rise in water temperature, however, took place in the latter half of 
March. The temperature passed 20° C. on March 21, and six days later was over 
25° C. The initiation of spawning as a response to rise in temperature is well illus- 
trated here, for the first larvae were found in the water on the 28th. However, instead 
of appearing immediately after the temperature reached 20° C., their appearance is 
more closely coordinated with the 25° C. level. This indicates that there is consider- 
able lag between the time the water temperature reaches 20° C. and the time spawning 
begins. In fact it is very probable that the surface water, in which the oyster reefs 
are located, reached 20° C. well in advance of the time that the average temperature 
of the deeper water, as shown in the figure, attained this level. 
The thermograph records show that even at a depth of 2 % feet the temperature 
was maintained for several hours at 20° C. as early as March 7. For 13 hours on 
March 9 it was between 20° and 22° C. On the 10th it was for 8 hours between 20° and 
21° C. From the 10th until the 20th the maximum temperature shown on the charts 
was well over 19° C. Since the spring rise in temperature, beginning early in March, 
was due naturally to air temperature and sunshine it is probable that throughout the 
early part of the month the temperature of the water at the level of the oyster beds 
was 20° C. or above during most of the daylight hours, yet no larvse were found in the 
water and the oysters appeared to be mostly unready to spawn. 
On the basis of these observations, it would be impossible to state that 20° C. is 
the critical temperature for spawning of oysters on the Gulf coast. However, Galtsoff 
(1930) showed experimentally that spawning will not occur below 20° C. but at this 
or higher temperatures it will take place when the specimens are adequately stimulated. 
From field observations on the Atlantic coast it was concluded by Churchill (1920), 
Nelson (1928), and Prytherch (1929) that spawning begins shortly after the water 
